Photographic recording of sound



Nov. 2, 1937. E. w. KELLOGG 2,097,668

PHOTOGRAFHIC RECORDING OF SOUND SSheets-Sheet 1 Filed Sept. 14, 1935 ATTORN EY Nov. 2, 1937. E. w. KELLOGG PHOTOGRAPHIC RECORDING OF SOUND Filed Sept. 14, 1935 5 Sheets-Sheet 2 a v z;

WA V5 INVENTOR [Dix/A50 1M fffLd66 BY M Z ATTORNE Nov. 2, 1937. E. w. KELLOGG 2,097,668

PHOTOGRAPHIC RECORDING OF SOUND Filed Sept. 14, 1935 5 Sheets-Sheet 3 INVENTOR 150116450 W KZ'ZL 066 ATTORNEY Patented Nov. 2, 1937 7 2,097,668 PHOTOGRAPHIC RECORDING OF SOUND Edward W. Kellogg, Moorestown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 14, 1935, Serial No. 40,557

16 Claims. (Cl. 179-4003) This invention relates to the photographic recording of sound, and has for its principal object the provision of an improved apparatus and meth od of operation for obviating the fogging or poor Fig. 3 indicates some of the various positions a triangular light beam may assume with respect to a light slit,

Fig. 4 is an enlarged detail of the record,

5 delineation which tends to occur at the boundary Fig. 5 illustrates the rounding effect produced 5 of a small unexposed record area largely surat the tips of the small record areas by fogging, rounded by an exposed area. Fig. 6 illustrates a light beam which is modified In the making of variable width photographic in contour to eliminate fogging and the resultant records over-modulation frequently occurs withdistortion,

out such serious consequences as to render the re- Figs. 7, 8, and 9 illustrate the improved type of 10 cording useless. The maximum modulation record produced by the modified light beam, which can be had without distortion calls for Fig. 10 illustrates the type of track produced widening the transparent part of the track to the when the base of the triangular light beam full possible track width as a maximum, and to passes across the light slit,

l5 zero width as a minimum. If the recording gal- Figs. 11 to 15 are curves illustrating the relal5 vanometer executes movements larger than suftion between input and output resulting from the ficient to attain this degree of modulation (which use of difierently spaced and differently shaped is usually described as 100% modulation) the light beams, tops and bottoms of the waves are cut ofi and Fig. 16 illustrates a form of light beam which 20 distortion results. There is no way of curing this avoids the type of distortion illustrated by Fig. 20

distortion. 10, and

Defleetions 0f t golvanometer yond e Figs. 17 to 22 illustrate in somewhat different point at which the track width becomes zero, reform the same phenomena as Figs. 11 to 15. sult in isolated clear areas, instead of the desired The recorder of Fig. 1 includes a lamp 30 from continuous strip of clear area of variable width. hich light passes through the aperture 3| of a 25 These isolated areas end in more or less sharp mask 32 and lenses 33 and 34 to the vibratable points Where t ary i es ome toget e mirror 35 of a recording galvanometer. Light reand intersect. If in the final record these points fleeted fro the mirror 35 passes through a lens were infinitely Sharp, e distortion Wou d e on y 36, the slit 31 of a screen 38 and lens 39 to the that which is inevitable with over-modulation, surface of arecord medium 4|]. Lens 34 produces 30 but h imp rf c i ns of the photogr phy r l an optical image of aperture 3| in the plane of in fogging in. or rounding ofi the Sha p P the screen 38. Movements of the mirror cause and the result is a more abrupt change in the this optical image or light spot to move up and light transmitted through the record when the down, thereby illuminating varying fractions of scanning beam ses his p in than would be the slit 31. The illuminated portion of slit 31 is 35 the Case With the Sharp p der these imaged on the film by lens 39, producing a fine conditions, the distortion is more objectionable, line of light whose length varies in accordanc giving p ll y harsh Sound- It S the p po e with the vibration of mirror 35, thereby exposof my inv to re he fo ing which ing a band of film which varies in width.

4 occurs at the point where the width of the clear Figures 2 and 4 show the appearance of a print 40 area falls to zero and thereby reduce the harmful of a record so made. It will be convenient to efieots of the over-modulation refer to the light spot, as a triangle, since, apart In Certain types o recording, as Wi l be e from over-modulation, a triangular shaped light ploined, the breaking p f he so n r ck in spot sufiices. The full width rectangular portion a series of isolated clear areas is not confined to at the top of the triangle plays no part in normal 45 Conditions of over-modulation, but a Similar recording, and the sharp extension of the vertex photographic effect occurs at the end of each of the triangle is the subject matter of this invenclear area and distortion results. This distorti i n m y be r v y the p i i n f the Fig. 3 illustrates the manner in which the moveprinciple of my lnvehtiohments of the triangular shaped light spot 4| 50 The invention will be better understood from varies the fraction or the slit 42, which receives the following description when considered in conillumination, from a minimum length with the neotion with the accompanying r win s. and i s triangle in the highest position to full length with scope will be pointed out in the appended claims. the triangle in the lowest position. The actual Referring to the drawings, idth of the slit 42 is less in proportion to the 55 1 is a d rammatic illustration of a 113- size of the triangle 4| than can be shown on a diacording apparatus to which the invention may be grammatic drawing. Therefore, the slanting pp intersection is not a material factor in the length Fig. 2 illustrates the type of sound record Proof the illuminated portion of the sl1t whlch may dlleed by the apparatus of be thought of as determined by the intersection 60 of the edge of the triangle with a single line at the center of the slit.

' tographic images however, are

If, while the film is moved continuously, the galvanometer vibrates without over-modulation, the exposed area of film varies in width in such a way as to produce a perfect picture of the light spot movement as illustrated in Fig. 4:. Should over-modulation occur so thatthe vertex of the triangle crossing the slit, the length'of the line of light on the film falls to zero. This brings the exposed area of film to a sharp point every time the vertex crosses the slit, as indicated in Fig. 7, in which, as in all of the illustrations of record 'wave forms shown herein, an ex-.

posed area in the initial recording is shown white, although in the'negative it would be black on a white background. The figures although the surface of the film may receive full exposure, the minute element of illuminated film imparts some of its light to adjacent areas, but

receives no light in-return, it'therefore receives less exposure'than an element well within the exposed area. .Itwill be obvious that some loss of. exposure of the negative due to this effect occurs at all'edges of exposed areas, but the loss becomes practically twice as great when the unexposed areas are on both sides, as at an acute angle. In making a print the point of the clear area is darkened in part due to the fact that the j negative at this point has had less exposure, and therefore transmits some light, and also by contribution of light from the more highly illuminated'areas on both sides. Thus, bothin the a recording and printing there is a tendency to round off the point of the exposed area. The resulting sound track is shown in Fig. 5, which illustrates the effect of over-modulation in which thdvertex of the triangle crosses the slit, but the base does not. i f

If over-modulation causes the base of the triangle of Fig. 3 to cross the slit, the illumination suddenly changes from full slit length to zero, and the record would appear as shown in Fig. 10. This produces much worse distortion than the rounding of the points, because the sudden change of light, in reproduction, is much greater. Such an efiect however, can be entirely avoided by anadequate' extension of the aperture 3i, or the addition of a rectangle on the base of the triangle, as, shown in Fig. 16. Crossingof the slit by the base of the' triangle occurs less irequently than crossing by thevertex, for the reason that it is usual to apply a bias to the galvanometer during periods of low modulation which causes it to vibrate about a'mean position close to the vertex, instead of about a mean position in which theslit is 50% illuminated. ,This practice of biasingis for the purpose of reducing ground noise. Although the bias is only supposed to be applied when the modulation is low and danger of over-modulation slight, the bias can not beinpossible.

stantly changed back to zero, in case of a sudden increase in modulation. Under these conditions the high modulation occurs'with thegalvanometer slightly biased and there may be frequent crossings by the vertex, but none by the base of the triangle. j

Since in making photographic sound records by means of an optical system of the type shown in Fig. l, occasional crossings of the slit by the vertex of the triangle are practically inevitable, it is important that the harmful effect on sound quality be minimized, and in accordance with my invention this is accomplished by extending the vertex of the triangular aperture 3! of Fig. 1', thereby producing a light spot'of the shape shown in Fig. 6. When the tip of such a light spot crosses the slit, the exposed areas of the negative have their points drawn out as shown in Fig. 8. Since the extreme vertical dimension of the light spot has been increased by the extension of the vertex, over-modulation which without this extension would result in crossings by the tip of the triangle, would in many cases not be suificient to cause the tip of the extension to cross. Under these conditions the illuminated area never becomes zero in width, but assumes the shape illustrated in Fig. 9. The nature of the distortion which occurs in the case of over-modulation is illustrated in Figs. 11, to 15. Galvanometer deflection is plotted horizontally and photocell light, which is the output, is plotted vertically. It is well known that for .distortionless reproduction this relationship should be a straight line over the range used, as illustrated in Fig. 11. Were there no fogging of the tips of the exposed areas, crossings of the slit by the vertex of the triangle would result in a break in the input-output line, such as is shown at A in Fig. 12 with corresponding breaks in the output wave shape shown at A. Fig. 12 also shows a break at B where the entire slit is illuminated and no further increase in light is Over mcdulation in both directions is shown in Fig. 12. In Figs. 13, 14, and 15, only over-modulation in the direction of zero slit illuminationeis shown. r

' ince the light cannot be modulated beyond'the zero point, oven-modulation inevitably involves a break in the curve. The effect of such a break however, is made worse by rounding the points as shown in Fig. 5. Fig. 13 shows at C (as compared with A in Fig; 12) the effect of the fogging on the input-output characteristic, and at C, the efiect on the output wave shape.

The eifecton the input-output characteristic and on wave shape, of the sharp extension of the triangular vertex '(with a resulting record like Fig. 8) is shown in Fig. 14. In this'figure a slight disturbance is indicatedat D to show the eifect of fo ging which may occur at the tip of the extension. The transition from the sloping to the horizontal portion of the characteristic would inevitably be more gradual than without the extension, and a slight abrupt termination as at D, due to fogging, if not completely elimihated, would be of much smaller magnitude, as indicated by comparing Fig. 14-; and Fig. 13. In fact, if the extension tapers gradually to a sharp tip, the characteristic would in general involve a smooth transition curve without any sharp breaks, as indicated at E, in Fig. 15. If the overmodulation is not sufiicient to cause the tip of the, extension to crossthe slit, the characteristic would always be likeFig. '15. i

If the output wave shapes corresponding to characteristic shown in Figs. 11 to 15 are analyzed, it will be found that for a pure tone input a characteristic such as shown in Fig. 11 will give a pure tone output;Fig. 12 will give a. series of harmonics or c-vertcriies which represent distortion;-Fig. 13 would give practically the series of overtones as Fig. 12, with the addition of a number of still higher harmonics or overtones, and it is these overtones of higher frequency which give the distortion the harsher sound. The output corresponding to Fig. 14 would also have a series of overtones, but they would be oi much smaller magnitude compared with the desired fundamental than those of 13, especially the overtones of highest frequency. The output wave corresponding to the characteristic of Fig. 15 with its smooth transition at E, would have less harmonics than those of any of the other figures (except Fig. 11) and the overtones of extra high frequency in particular would be almost completely eliminated. The effect on the reproduced sound would be to make the effects of overmodulation much less noticeable, and much less irritating.

The objectionable effect of over-modulation resulting from the base of the triangle crossing the slit may also be reduced by an expedient similar to extending the vertex, namel by rounding the corners 48 of the aperture 3! of Fig. 1 where the end of the rectangle intersects with side of the triangle. Thus, both extremes or" overmodulation a smooth transition curve, like that on Fig. 15, would be introduced between the slcp ing and horizontal parts of the characteristic, instead of a sharp break ike that of Fig. 12. The advantage of such a rounding of the corners in case, is not because of fogging of the film. In fact, the tendency of fogging is to reduce the sharpness of the break in the characteristic when the base crosses the slit.

My invention may be applied with special ben fit to the recording of sound by means of a double track system which is the subject of a copending application of Glenn L. Dimmick, Serial No. 32,734, filed July 23, 1935 (Docket 10,463). The light beams which pass through the two tracks illuminate two photocells, the outputs of which are difierentially connected. The net photocell output current under these conditions is proportional to the algebraic difference between the light transmitted t ough the two sides of the track. With this arrangement each track might be a complete record of the waves, recorded in opposite phase, so that the widest part of one track is opposite the narrowest part of the other. This would constitute a push-pull system corresponding to what is commonly described as a Class A push-pull system in electrical amplifiers. In the system as applied however, one track is employed to register only the swings of the galvanometer in one direction from the eutral position, while the other track records only the swings in the opposite direction. The complete recording of a wave therefore, calls for the cooperation of the two sides of the track. In the system covered in the aforementioned application, the two sound tracks are recorded by means of two triangular shaped apertures whose images are arranged in relation to the slit as shown in Fig. 17. ment is the same as shown in Fig. 1, but the mask 32 in Fig. 1 is replaced by one having two cpenings instead or one. The two light spots, 53 and 44 on Fig. 17, move up and down together. They The galvanometer and lens arrange should be so adjusted and positioned that both vertices touch the middle line of the slit when the galvanometer mirror is in neutral position. When the galvanometer vibrates the exposed areas take the forms indicated at 45 and 45. The resulting output wave is found by plotting the algebraic diiference between the widths of clear areas 45 and 45, considering 45 as positive and 46 as nega. tive. Such an output wave is plotted and shown at 41. If the aperture images are not correctly placed, so that for a small range of movement neither vertex crosses the middle of the slit, there will be gaps in which there is no light transmitted through either track, and the resulting wave shape will be as shown at 49 in Fig. 18. Fig. 19 shows the effect of overlapping in the triangle positions. In this case there is a small region Within which some light is transmitted through both sides of the track and in plotting the resulting wave, the difference between the widths of the two openings is the ordinate. Since in Fig. 19 the overlap is the result of incorrect location of the triangles, wave shape distortion results, the output wave being excessively steep at the points where it crosses the zero line.

In the push-pull system overlap in the positions of the tips of the apertures is permissible, provided the triangles themselves are correctly positioned, and the algebraic difference in the wi the of the two aperture extensions when plotted against position, is a straight line. Thus, in Fig. 20 the triangles themselves are located as in Fig. 17, but the vertex extensions result in slight overlaps of the exposed areas. When the algebraic difference is plotted, an undistorted wave results. In the shaping of these extensions, allowance may be made for the effect of fogging at the extreme tip, the shape being worked out experimentally to produce minimum distortion. Overlapping of the negative and positive waves, and the production of an overall characteristic free from distortion, is .analagous to the superposition of the characteristics of two oppositely connected vacuum tubes in what is commonly described a Class B amplifier.

The push-pull system which I have just described has certain important advantages over the usual single track system, especially in the reduction of ground noise during periods of low modulation, but it is obvious that this system involves a great many crossings of the slit by the tips of the triangular apertures, and on each such crossing wave shape distortion may occur due to fogging. The importance of vertex extensions is therefore much increased by the employment of such a push-pull system. Where the ends of the transparent areas are rounded by fogging, as shown in Fig. 5, no adjustment of triangle position can eliminate or neutralize the breaks in the: resulting curve, which would have the appearance indicated at 50 in Fig. 21. With the points of the clear areas drawn out, by the employment of the vertex extensions in the mask, becomes quite easy to obtain .a substantially linear relation between light spot position and output.

Fig. 22 shows a form of vertex extension especially adapted to the push-pull system. The light is never entirely out off from either track, and there is no question of a possible final sharp tip where fogging could produce even a slight abrupt change in transmitted light.

In the following appended claims, the terms triangular aperture or triangular light spot or equivalent terms are employed to designate apertures or light spots whose form is based on sound to be recorded, and applying said modulated beam to said record;

2. In an apparatus for producing graphic sound record having relatively narrow unexposed areas, the combination of means for producing a light beam so shaped as to prevent fo ging of said narrow areas, means for modulating said beam in accordance with the sound to be recordedand means for applying said modulated beam to said recorder.

3. In an apparatus for producing a *photographic sound record havingunexposed areas the boundaries whereof are close together, the combination of means including a mask provided with a' triangularly-shaped aperture extended at its apex to prevent fogging of said areas,'means for modulating said beam in accordance with the sound to be recorded, and means for applying said modulated beam to said recorder.

4. In an apparatus for producing. a photographic sound record having relatively narrow unexposed areas, the combination of means including a triangularly-shaped aperture shaped at its apex to prevent fogging of said areas, means for modulating said beam in accordance with the sound to be recorded,'and means for applying said modulated beam to said recorder.

' 5. In anapparatus'for producing a photographic sound record having narrow unexposed areas, the combination of means for producing alight beam having an edge'so formed as to prevent fogging of said areas, means for modulating said beam in accordance with the sound to be recorded, and means for applying said modulated beam to said recorder. a g

6. In the production of a variable area sound record of the photographic type,'the method of minimizing distortion which includes maintaining a residual minimum of exposed area throughout the record length whereby the fogging effect otherwise produced at relatively narrow record areas is eliminated.

'7. In the production of a variable area'sound record of the photographic type, the method of minimizing distortion which includes retarding the rapidity with which the exposed area of said record approaches zero whereby the fogging ef- .fect otherwise produced at relatively narrow record areas is eliminated. 8. In an apparatus for producing variable width records of thephotographic type, means for producing an exposure area which varies in accordance with the signal to be recorded, and'means for maintaining the width of said area above a predetermined minimum whereby the fogging effect otherwise produced a relatively narrow record areas is eliminated.

9. A variable width photographic record in a photowidth of the exposed area approaches zero. 7

13. In a system for making variable width which the transparent areas which correspond to the recorded waves are prolonged by narrow extensions in the direction of motion of the film, whereby wave-shape distortion due to fogging of the photographic emulsion is reduced. 7 V

10. In apparatus for making variable width photographic records a mask with an aperture 'having two substantially intersecting edges, and

means for prevention of distortion due'to fogging-comprising an extension of said aperture at the point of intersection of said edges.

11. In apparatus for making variable width photographic records means for producing a nonlinear relationship between input to said apparatus and the width of the exposed area of said record, whereby the rate of change of said width with respect to input, is gradually reduced, as a limiting value is approached.

12. In apparatus for making variable width photographic records, means for producing a nonlinear relationship between input to said apparatus and the width of the exposed area of said record, whereby the rate of change of said width with respect to input, is gradually reduced as the photographic records, a galvanometer, means establishing a relationship between the deflections of said galvanometer and the width of the exposed area of the record, said relationship being substantially linear throughout the major portion of the possible range of modulation, and. nonlinear near a limit of modulation, such that the approached more grad between their sides.

15. Sound recording apparatus comprising a light source, an aperture plate for defining a beam of light, a galvanometer for deflecting the beam of light in accordance with sounds to be recorded, a slit, means for focusing said beam on said slit to form an image of said aperture and means for focusing the light passing through the slit on a v film, the said aperture plate being provided with opposed triangular apertures having their vertices extended a distance not less than the altitude of the triangle. 7

16. Sound recording apparatus comprising a light source, an aperture plate for defining a beam of light, a galvanometer for deflecting the beam of light in accordance with sounds to be recorded,

a slit, means for focusing said beam on said slit to form an image of said aperture and means for focusing the light passing through the slit on a film, the said aperture plate being provided with opposed triangular apertures having their vertices extended a distance not less than the altitude of the triangle, the said extension having sufiicient width to prevent fogging in of the corresponding clear portion of the positive print. EDWARD W. KELLOGG. 

